Optical devices of a genre referred to as photonic crystal have been attracting attention in recent years. Yablonovitch: E. Yablonovitch “Phys. Rev. Lett.” Vol. 58, p. 2059, 1987 describes that the development of photonic crystals is centered on a technique of producing a periodic distribution of refractive index by forming a periodic structure for an optical material and effectively utilizing the behavior of light in the specific refractive index distribution and a technique of effectively utilizing a phenomenon that light emitting modes can be controlled when a light emitting material is placed in a specific refractive index distribution. The potential of these techniques as applied to optical devices are being discussed.
Relating to the optical device techniques, so-called DFB lasers realized by effectively utilizing a one-dimensional periodic structure for semiconductor lasers have been put to actual use. Such optical devices are produced by applying one-dimensional photonic crystal. At present, efforts are equally being paid for basic research activities in the field of applying two-dimensional photonic crystal having a two-dimensional structure of cylindrical holes that are periodically arranged in a plane to components of optical communication devices. Furthermore, three-dimensional periodic structures referred to as three-dimensional photonic crystals are also known.
Of photonic crystals, two-dimensional photonic crystals are attracting particular attention for several reasons including that they provide a high degree of freedom and hence can be made to operate in a sophisticated way if compared with one-dimensional photonic crystals and that, on the other hand, they can be prepared relatively easily if compared with three-dimensional photonic crystals by utilizing the known semiconductor processing techniques. Basic research activities are in progress for the purpose of developing various devices using two-dimensional photonic crystal. Such devices are highly promising for finding practical applications.
Devices of different types using two-dimensional photonic crystal are objects of current researches. They include micro-waveguide circuits, wavelength filters and micro-lasers to be used as optical communication devices. Kawakami et al., “Photonic Crystal Technology and its Applications” (CMC Publishing, 2002), pp. 252, 257 and 258 describes such devices.
As for the field of the biotechnology and the related industry that has been growing remarkably in recent years, the applicant of the present patent application has proposed “a Micro-Sensor Using a Micro-Resonator Laser” in Japanese Patent Application No. 2002-299153 as an application of two-dimensional photonic crystal to a highly integrated and highly sensitive biosensor chip.
Of two-dimensional photonic crystals that are objects of researches for the purpose of actually developing devices, two-dimensional slab type photonic crystals have been prepared most numerously. The slab type refers to the one in which light is confined in a direction not showing any periodic structure by sandwiching a high refractive index core layer between low refractive index clad layers so that light is confined to the high refractive index core layer for propagation.
The thickness of the slab, or the core layer, is related to the conditions on which an electromagnetic wave mode of light can exist in the direction of the thickness. Particularly, in a case where only a single mode can exist, the optical path length obtained by multiplying the slab thickness by the refractive index is approximately about a half of the wavelength. Thus, the optical path length of a single round trip is approximately equal to the wavelength. In other words, this provides the smallest thickness for allowing light that makes a single round trip to interfere with light that makes several round trips so as to raise the intensity of light. In reality, the thickness is computed by taking the propagation of light to the clad layer into consideration (see Koshiba, “Optical Waveguide Analysis”, 1990, Asakura-Shoten)
SOI wafers formed by using an SiO2 layer (BOX (buried oxide) layer) that is formed on an Si substrate as clad and forming an Si layer (SOI (silicon on insulator) layer) thereon as core have been attracting attention in recent years as two-dimensional slab type photonic crystals (see Notomi, “Applied Physics”, Vol. 72, No. 7, 2003, “Photonic Crystal Slabs Using SOI Slabs”).
The use of such an Si type material provides advantages including (1) the currently available SOI wafer preparing techniques are already feasible so that the necessary precision level can be attained and (2) the currently available sophisticated Si process techniques can be applied to forming a periodic pattern on an SOI layer to be used for a core layer.
Other areas of utilization of SOI wafers for two-dimensional (2D) slab type optical devices include Si fine wire waveguides. As in the case of two-dimensional slab type photonic crystals, researches and developments are under way for confining light to micro-waveguides of 1 μm or less and realizing curved waveguide devices with a small radius of curvature by utilizing a large refractive index difference between Si and SiO2 (see, above-cited Kawakami's paper, p. 252).
However, when such an SOI wafer is used for a 2D slab type photonic crystal or a fine wire waveguide, it is necessary to use relatively thick BOX layers typically having a thickness of 1 μm or more because of the requirements to be met for confining light. When light is confined to a core layer, it can propagate into the clad layer as pointed out above and, if the clad layer is thin, propagating light of the evanescent mode is coupled with light of the radiation mode directed to the substrate to give rise to a radiation loss in a direction toward the substrate. The above cited Kawakami's paper, pp. 257 and 258, describes a calculated thickness necessary for the BOX layer when the allowable loss is −40 dB.
To prepare an SOI wafer comprising BOX layers having a thickness of 1 μm or more, a so-called bonding technique needs to be used. Such techniques are described in Celler and Yasuda, “Status Quo of SOI Wafers for MEMS”, 2002. 5, Electronic Technology and also in Iyer and Auberton-Herve, “SILICON WAFER BONDING TECHNOLOGY for VLSI and MEMS applications” (EMIS PROCESSING—SERIES 1, ISBN 0 85296 0395, 2002, The Institution of Electrical Engineers.
However, a process of bonding wafers inevitably includes a bonding step along with a seed wafer cutting step and a plurality of starting wafer preparation steps including special steps such as an H+ ion implanting step and other complex steps. Consequently, the structure of the device to be prepared on such a wafer and the device preparation process are very special if compared with ordinary Si wafers. Then, the prepared SOI wafer is very expensive so that the applications of such wafers are limited only to semiconductor logic circuits such as CPUs having a high added value that makes the use of such wafers economically feasible.
This invention is intended to dissolve the above identified problems of the background art and it is an object of the present invention to provide a highly functional high precision optical device realized by using two-dimensional slab type photonic crystal or a fine wire waveguide having a porous layer as clad.
Another object of the present invention is to provide a method of manufacturing a highly functional high precision optical device having a large area and realized by using two-dimensional slab type photonic crystal or a fine wire waveguide at low cost.